Testing circuit



June 21, 1960 H. N. NERWIN 2,942,178

TESTING CIRCUIT Filed July 12; 1957 Fig-1 LL CIRCUIT SIGNAL DETECTOR SOURCE CIRCUIT 22 I H K 36 F1 g. '3 l 42 44 =3 27 fi'Zz Efi [UP 39 HENRY lV. IVER wnv United States] y 2,942,178 TESTING CIRCUIT Henry NJ Nerwin, Chicago, Ill., assignor to Magnaflux Corporation, Chicago, 111., a corporation of Delaware Filed JulylZ, 1957, Ser. No. 671,566

2 Claims. c1. 324-40 The present invention relates in general to testing circuits and more particularly concerns an automatic checking circuit for use with an eddy current inspection or testing system utilizing a balanced transformer detecting systern during ;an interval between testing articles,a controlled, unbalance is automatically introduced into the system for checking detector. operation, If the detector fails to indic'ateanunbalance during this interval, an alarm condition is automatically indicated. Thus, the system is frequently-monitored for proper operation without interfering with the normal testing routine.

An eddy current testing system is especially useful for detecting flaws in metallic structures, such as cylindrical pipes.' In a typical system of this type, the pipe under test is continuously passed through the center of each of a'pair of balanced transformers axially spaced relative to the pipe-axis. The primaries of the transformers are serially-connected and energized with an alternating curre nt signal. The secondaries are serially-connected in a sense whereby the signal derived across each inresponse to the alternating signal applied across the primary wind ings tend to cancel. When the magnitude of the two, sec ondary signals are equal, no signal appears across the serial combination. Since both transformers are con structed insubstantially the same manner, when there is no pipein the gaps, the resultant signal derived across the secondaries is substantially zero. This condition is retained when the pipe passes through transformer coilcenters as long as the characteristics of the pipe region passing through one transformer coil center. is substantially thesame as the region passingthrough the other since eddycurrent losses load each transformerto the same extent. However, when there is a flaw in the pipe wall there will be a change in the eddy 'currentt losses introduced by that portion of the' wall and the signal-derived across the secondary of the associated trans former is altered accordingly, thereby introducing an unbalance which results in asignal across the serially-connected secondaries of magnitude related to the degree of unbalance.

Should the signal to the primary be interrupted or an open circuit develop in any of the "windings, then the signal derived across the serially-connected secondary windings would continue to be zero, even if a flaw were then passing within one of the transformer centers.

The present invention contemplates and has as a pri mary object the provision of means for automatically testing the erroneous indication of an output signal without'interrupting the routine inspection of the eddy current testing system. c

Another object of the invention is the provision of means for giving an alarm indication when improper operation of the detecting system is sensed.

Still another object of the invention is the achievement of the foregoing objects by the addition of a few relatively inexpensive components to an existing eddy current testing system.

, 2,942,178 i ,Fatente'd i-June 2 1 1969 According to one. aspect of the invention, an additional winding is added to one of the transformers. In response to an indication that inspection of the entire length "of pipe is complete, means are provided for connecting a selected resistance across the additional winding, thereby loading the associated transformer and imparting a selected degree of imbalance to which the detector circuit responds, if operating properly. If thedetecting circuit fails to respond to the unbalance thus introduced, means are provided for actuating an alarm indication.

In another form, of the invention, the desired unbalance is effected by selectively connecting a resistor across one of the secondary windings.

Other features, objects, and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which; i i

Fig. 1 is a combined block-pictorial diagram illustrating the relation of the pipe being inspected to the trans-- formers and the photoelectric cell system which indicates:

ance obtained by selectively connecting a resistor across one of the secondary windings.

With reference now to the drawing and more particularly Fig. 1 thereof, there is illustrated a pipe 11 moving along a direction indicated by arrow 12 for scanning by balanced transformers 13 and 14. A light source 15 .energizes a photocell 16 when the end 17 of pipe. 11 no longer interrupts the path of light between light source.- 15 and photocell 16. Photocell circuit 21 provides an output signal on terminal 22 when photocell 16 is energized, thereby indicating the end of scanning pipe 11 for defects. H

Having described the physical relationship of the transformers in a typical-system the means forindicating inspection of a length of pipe is complete the electrical circuit arrangement which. enables, the operation of the detector circuitto beautomatically checked during the intella'val a pipe 18 not being scanned for flaws will be descri ed. i V

A description the circuit arrangement will precede a discussion of its mode of operation. Referring to Fig. 2 there is illustrated a combined block-schematic circuit diagram of such a system. j Indescribing Fig. 2 as well as Fig. 3, reference numerals which designate corresponding elements in previously described portions of the drawings are retained. a

The primaries 23 and 24 of transformers l3 and 14 respectively are serially-connected and energizedlwith an alternating current signal from signal source 25. The secondaries 26 and 27 respectively of the latter transformers are serially-connected in the sense indicated by the dots and the ends opposite the common connection are connected to the input of detector circuit 28.

In Fig. 2' the relays are represented in-their normal position with-the associated solenoid deenergized. The

output of detector circuit 28 is connected through relay arm 31 when the associated solenoid 3-2 is energized, the

solenoid 32 being energized by a signal from outputtermioperation will be discu'ssed. .During normal operation as a flaw d cto s gna sou e 2 emits 3 .1 alts 8 current signal which, in the absence of a flaw, ind ces voltages of equal magnitude and opposite phase across secondary w nd -Z6 and h s d stw rv t Z8 .and the voltage across thesecondary of'the transformer proximate to the flaw changes accordingly, thereby producing an unbalance which is sensed by detector circuit 28 to provide an output signal which is indicative of a nun; The particular form of detector circuit2 8 is not va part of'this invention and may, for eiiample, inclnde .a voltage sensitive relay which 'moyesitsassociatd' arm position when the magnitudeofyoltage across the associated solenoid exceeds a predetermined level.

. According to the invention, once scanningofthe pipe for flaws is complete,v a predetermined degree of 'unbal ance is deliberately introduced and failure of the detector circuit '28 to respond to this unbalance is indicated by an alarm response. Referring again to Fig. 1, when the scanning of the pipe llis complete, the line of transmission betweenlight source'lS and photocell 1 6 isno longer interrupted, photocell 16 is energized and'photocell circuit 21 provides an output potenti'al on terminal 12 indicative thereof.

Referring to Fig. 2, the potential, on terminal ZZ energizes solenoid 32, thereby closing arms 31, 33 and 41. 1 s c s r l y a m 4 connec he var ble e ance 38 across the additional winding 37 to introduce ddi na os e t t an o me whe b the ma ni u f vo e across ewfida 1 7 i ss tha a a es secondary 26. The unbalance thus produced is sensed by detector circuit @8,'when operating normally, to provide a'potential which is coupled through relay arm '31 to energize solenoid 39, thereby opening relay arm 35 arid preventing the potential across batter-37134 from activating an indication from alarm 36. Should detector circuit 28 fail to respond to the unbalanceintroduced, thensolenoid 39 is not energized, relay arm 35 remains closed and the potential from battery 34 coupled to alarm 36 activates an alarm indication from the'latter.

Variable resistance 38 can be adjusted to provide substant ially any desired degree of unbalance." Normally, this adjustment is made to'provide an unbalance which corresponds with the desired minimum degree ofunbalance for producing a response -fr'oni"detec'tor circuit -28. When a new pipe is inserted forv testing, the path between light source 15 and photocell 16 is interrupted; solenoid -32 is deenergized andarms 31, 33 and 41 opened, thereby disabling the testing circuit.

With-reference toFig. 3 there is illustrated a variation ofthe circuit of-E'ig. 2 in which all the advantages "of the lattercircuit are retained, yet no additional winding is required, thereby permitting the automatic checking circuit to be connected to an existing eddy current testing systernj Primaries 23 andf24 andsecondaries 26 and 27 are serially connected as in Fig. 2. An alternating currentsig'nal is applied from signal source 25 across terminals42 and 43'and terminals 44' and 45 are connected to the input of detector circuit 28. However, instead of variable resistance SS-being connected across an addi- 'tional'winding' when relay'arm 41' is closed, variable resistance 381s directly connected across secondary-winding '27 to introduce the desired unbalance withoutrequii'ing the use of an 'additionalwinding.

lt is importan't'thatthe photocell be so located "that thetesting operation is initiated while the pipelies'within both transformers, since a large degree of unbalance is produced when the end of the pipe passes the transformer 13, before reaching the transformer 14 and such a large degree of unbalance would provide no criterion as tothe sensitivity of the detector circuit. 'It may be further noted that the photocell 16 may be part of a photocell assembly used to permit functioning ofthe detector circuit only when the pipe lies within both transformers.

Thus, by adding a'fewrelativclyinexpensive additional components, a malfunctioning of the detecting system may be obtained without interrupting the normal operation of the eddy current testing system. Furthermore when a malfunction is detected, an alarm is automatically activated.

The specific embodiments described herein are exemplary only. It is apparent that those skilled in the art may make numerous modifications of and departures from these specific structures without departing from the inventive concepts. Consequently, the invention is'to bc construed .as limited onlyby the spirit and scope 'of'the aar...sss c a ms claim as rnyinvention:

1. In a system for sensing variations in a physical characteristic of a test piece, an electrical test unit arrainged to be disposed in a position adjacent said test piece for producing an output signal proportional to deyiations in a physical characteristic of the test'piece from a certain value, a detectorcircuit responsive to said output signal, sensing means responsive to movement of said r runs relative to said tes't'p'iece to a position away from said position, an alarm device, a first circuit operable to produce a certain output signal from said unit, a second circuit operable to apply an energizing signal to said alarm devicepa third circuit :for preventing operation of'said alarm device in response to an output signal fromsaid detector circuit of predetermined magnihide, and means controlled by said sensing means for simultaneouslyrendering said first, second and'third cirsuits oper ble. I v

' '2. In a system for comparing the physical characteristics of test pieces a source ofialternati-ng current, primary transformer winding means connected to said sourceand inductively coupled to both of saidztest pieces to induce alternating magnetic fields in regions occupied by said test pieces, a pair" of second winding means respectively coupled inductively to said test pieces. to produce induced alternating electromotive forces proportional to the strengths of said magnetic fields, an indicating circuit, series circuit means connecting said secondarywindingsin phase-opposition to said indicator circuit to produce an References Cited in the file of this patent N ED A S ATENTS 851 81 e -e 32 2,074,742. Drake Mar. 23; 1937 

